Brake Specific Fuel Consumption (or
BSFC) is the ratio betweenthe
engine's fuel mass consumption and the crankshaft power it
isproducing. This makes it both a
valuable fuel efficiency indicator
and one more useful tool in gauging an engine's
state-of-tune.

In the USA, the fuel flow for BSFC
calculations is normallyexpressed
in pounds per hour (lb/hr) while the output units, ofcourse, are in horsepower (Hp). So, our standard
formula for BSFCbecomes: lb/Hp-hr
. For an engine producing 200 horsepower,while guzzling 100 pounds of gasoline per hour, the
equationwould yield 100/200 =
0.50 BSFC. Unlike AFR (Air/Fuel ratio)readings which only reveal the mixture, BSFC data
represents thepower provided per
fuel unit. AFR and BSFC are not equivalent!

The beauty of BSFC #'s are that they
remain similar over a widerange
of engine sizes (assuming both are of similar mechanicaldesign and compression ratio). For example, a tiny
one cylinder50cc four-stroke and
a 454ci V8 might both have a BSFC ratio of0.45 lb/Hp-hr (when optimally tuned) at their
respective peaktorque points.
Thus, if either of these engine's were overly richened,its BSFC might climb into the 0.55 to 0.65 range
(because the fuelflow in our
equation will be going up as the power is going down.

Consider the above example on a dyno
that only displayshorsepower.
Without the BSFC data, it is harder to know that a richmixture engine is the reason for sub-par output. But,
when you alsohave the BSFC
numbers in front of you, it aims you at the cause.

BSFC values all follow a hooked curve.
At idle they run muchhigher - due
primarily to the closed throttle pumping losses andexcessive camshaft overlap. Minimum BSFC #'s occur at
about thepeak torque operating
rage - the most fuel efficient (per Hp)operating point for an engine. As RPM increases
towards peakpower, the BSFC rises
again, since more fuel energy is consumedjust overcoming the speed induced friction and
breathingrestrictions.

Below are sample BSFC ranges for
several typical engine types.
Note, these are provided to illustrate relative BSFC
behavior only.Realize that any
change that improves the mechanical efficiency ofthe engine (e.g. a dry sump oil pan, electric water
pump, lowtension rings, lighter
oil, etc.) will also reduce its BSFC values!

To find the optimum fuel curve for an
engine running on the dyno, you should experiment with
richer and leaner air fuel mixtures at incremental steady
state test points (e.g. every 250 RPM). Starting with a
known safe AFR value (rich fuel map, jet, or mixture needle
setting) record the Hp, temperature, and BSFC values. Lean
down slightly and retest – you are looking for improved
power.

Keep in mind that you can easily go too
far and end up sticking a piston, etc. This is why you must
listen for knocking while monitoring the EGTs, AFR, and BSFC
values. Experienced tuners will push the envelope a bit to
get a flash Hp reading – but then back off for a cooling
period - before the thermal heat sink protection is lost.

Once the peak safe power mixture is
determined for each RPM point you can plot a target BSFC
curve for the engine combination. Actually duplicating that
shape is easy with a mapped fuel injection system. For
carburetor equipped applications you normally need to work
out air emulsion tube air bleed sizes, tweak boost venturis,
etc. and even then some compromises must be accepted.

Back on the road, dynamometer optimized
curves often prove to be a bit too lean for a good driving
feel. This is because during transient conditions, like
moving the throttle or accelerating, mixture requirements
change. Since a slightly richer mixture only depresses power
a tiny amount, whereas a slightly lean mixture can cause a
very noticeable miss-fire and bog, the best drivability
compromise is usually to richen up a tiny bit. That way,
objectionable lean bogs are avoided at only a small expense
of power and fuel economy.

The overall dynamometer verified fuel
curve is still valid; it’s just been made more tolerant with
a little fattening. As you gain tuning experience with
various engine and induction system combinations, you will
be able to hit an acceptable derivability mixture with
little or no road test time. Compare that with blindly
flogging the engine to death on the track – trying to
optimize the best power curve mixture.